Intra-abdominal myositis ossificans, also known as heterotopic mesenteric ossification, defines the formation of bone-like lesions inside the abdominal cavity. It is a rare medical condition, usually following abdominal surgery or trauma. A 55-year-old male presented for closure of a Hartmann's colostomy, created 6 months ago because of sigmoid adenocarcinoma. Intraoperative findings consisted of an elongated bone-like lesion attached on the peritoneum and protruding inside the abdominal cavity, while two more stiff calcified nodules were found in the mesocolon. All lesions were excised. The presence of a hard consistency sigmoid stump led to cancelation of the operation. Heterotopous bone formation constitutes a challenging surgical condition, since its clinical presentation is innocuous and repetitive surgery acts a stimuli for additional bone creation. Computed tomography scan plays a major role in its diagnosis, while nonsteroidal anti-inflammatory drugs and cimetidine are helpful in preventing further recurrence of the disease.